This study has two major goals: 1) to provide new evidence on the nature of the relationship between habitual dietary fat intake and the development of coronary heart disease (CHD), and 2) to examine the possibility that the association between low serum cholesterol level and mortality from cancer or stroke is related to habitual dietary fat composition. We propose to analyze the cholesterol ester fatty acid content of the baseline sera of selected subjects chosen from the cohort of 12,866 men who have been followed for 6-8 years in the Multiple Risk Factor Intervention Trial (MRFIT). We will compare the results in incident cases of CHD, cancer, and stroke (127 subjects per group) with those of matched control subjects, to test whether this objective measure of dietary habits predicted the later onset of each disease. The special features of this study are: 1) The rigor and feasibility of the design, a case-control cohort approach that combines the high internal validity of a longitudinal study with the economy in resources and time of a retrospective study; 2) The validity of the main independent variable: the fatty acid composition of cholesterol esters, an objective and reliable measure of dietary habits, will be assessed by a laboratory with experience in these methods on sera that have remained at -55 C throughout their 6-8 years of storage and are known, through testing for peroxidative damage, to be in excellent condition; 3) The availability of the full MRFIT data bank on other independent variables, with complete and accurate information on dietary, physiological and psychosocial variables collected over the 6-8 years of the Trial; 4) The validity of the dependent variable: morbid and mortal events classified by the standardized MRFIT procedures; 5) The importance of the research question which seeks new and direct evidence that will help resolve one of our most pernicious controversies in the health policy arena: How the adoption of a fat-controlled diet by all Americans would influence their commonest causes of mortality, CHD, cancer, and stroke.